Fix 'ballot' and 'compiling' browser tests under nightwatch_local

Running `npm rpn nightwatch_local`, a few tests would consistently fail
due to the "Account" dropdown in the Run Tab (i.e. 'select#txorigin') being
filled-in asynchronously and the tests were running without a valid
account value.  E.g. contract deployment would fail with:
"creation of Ballot errored: Invalid account selected"

This patch fixes it by adding a, admittedly weird looking,
`waitForValue()` function which checks the `value` attribute
for some DOM element is present.

Browsing the nightwatch API, I expected:
`browser.expect.element(selector).value` to provide equivalent
functionality but empirically I couldn't get that to work.

Tested with:
```
nightwatch 0.9.21
selenium-server-standalone 3.9.1
ChromeDriver 74.0.3729.6
Chrome 74.0.3729.169
```

Error message on test failure:
```
 - Ballot (13.92s)
   Timed out while waiting for element <.instance:nth-of-type(2)> to be present for 10000 milliseconds.  - expected "found" but got: "not found"
       at runTests (/home/scottt/work/remix-0.8/remix-ide/test-browser/tests/ballot.js:40:8)
       at Object.Ballot (/home/scottt/work/remix-0.8/remix-ide/test-browser/tests/ballot.js:19:5)

 ✖ compiling
   - Compiling (13.389s)
   Timed out while waiting for element <.instance:nth-of-type(2)> to be present for 10000 milliseconds.  - expected "found" but got: "not found"
       at /home/scottt/work/remix-0.8/remix-ide/test-browser/tests/compiling.js:43:8
       at getCompiledContracts (/home/scottt/work/remix-0.8/remix-ide/test-browser/helpers/contracts.js:84:5)
       at Object.<anonymous> (/home/scottt/work/remix-0.8/remix-ide/test-browser/helpers/contracts.js:50:5)
```

test-browser/helpers/contracts.js

@@ -26,7 +26,8 @@ module.exports = {
   getAddressAtPosition,
   clickLaunchIcon,
   scrollInto,
-  signMsg
+  signMsg,
+  waitForValue
 }
 
 function clickLaunchIcon (icon) {
@@ -467,3 +468,27 @@ function goToVMtraceStep (browser, step, done, incr) {
     }
   })
 }
+
+function waitForValue (selector, retryCount) {
+  function performFunc (browser, done) {
+    browser.getValue(selector, function (result) {
+      const value = result.value
+      if (value) {
+        done()
+      } else {
+        if (retryCount === 0) {
+          done()
+        } else {
+          retryCount--
+          browser.pause(500).perform(function (browser, done2) {
+            performFunc(browser, function () {
+              done2()
+              done()
+            })
+          })
+        }
+      }
+    })
+  }
+  return performFunc
+}

test-browser/tests/ballot.js

@@ -36,7 +36,8 @@ function runTests (browser, testData) {
     })
   }).clickLaunchIcon('run')
       .setValue('input[placeholder="uint8 _numProposals"]', '1')
-      .click('#runTabView button[class^="instanceButton"]')
+      .perform(contractHelper.waitForValue('#txorigin', 10)) // browser.expect.element('#txorigin').value somehow doesn't work
+      .click('#runTabView button[class^="instanceButton"]')  // "creation of Ballot errored: Invalid account selected"
       .waitForElementPresent('.instance:nth-of-type(2)')
       .click('.instance:nth-of-type(2) > div > button')
       .testFunction('delegate - transact (not payable)', '0x0571a2439ea58bd349dd130afb8aff62a33af14c06de0dbc3928519bdf13ce2e',

test-browser/tests/compiling.js

@@ -39,6 +39,7 @@ function runTests (browser) {
 function testSimpleContract (browser, callback) {
   contractHelper.testContracts(browser, 'Untitled.sol', sources[0]['browser/Untitled.sol'], ['TestContract'], function () {
     browser.clickLaunchIcon('run')
+      .perform(contractHelper.waitForValue('#txorigin', 10))
       .click('#runTabView button[class^="instanceButton"]')
       .waitForElementPresent('.instance:nth-of-type(2)')
       .click('.instance:nth-of-type(2) > div > button')
@@ -70,6 +71,7 @@ function testSimpleContract (browser, callback) {
 function testReturnValues (browser, callback) {
   contractHelper.testContracts(browser, 'returnValues.sol', sources[1]['browser/returnValues.sol'], ['testReturnValues'], function () {
     browser.clickLaunchIcon('run')
+      .perform(contractHelper.waitForValue('#txorigin', 10))
       .click('#runTabView button[class^="instanceButton"]')
       .waitForElementPresent('.instance:nth-of-type(2)')
       .click('.instance:nth-of-type(2) > div > button')
@@ -108,6 +110,7 @@ function testReturnValues (browser, callback) {
 function testInputValues (browser, callback) {
   contractHelper.testContracts(browser, 'inputValues.sol', sources[2]['browser/inputValues.sol'], ['test'], function () {
     browser.clickLaunchIcon('run')
+        .perform(contractHelper.waitForValue('#txorigin', 10))
         .click('#runTabView button[class^="instanceButton"]')
         .waitForElementPresent('.instance:nth-of-type(2)')
         .click('.instance:nth-of-type(2) > div > button')
@@ -160,8 +163,8 @@ function testInputValues (browser, callback) {
 
 var sources = [
   {'browser/Untitled.sol': {content: `
-      contract TestContract { function f() public returns (uint) { return 8; } 
-      function g() public returns (uint, string memory, bool, uint) {  
+      contract TestContract { function f() public returns (uint) { return 8; }
+      function g() public returns (uint, string memory, bool, uint) {
         uint payment = 345;
         bool payed = true;
         string memory comment = "comment_comment_";
@@ -176,7 +179,7 @@ var sources = [
         _i = -345;
         _a = msg.sender;
     }
-    
+
     function retunValues2 () public returns (byte _b, bytes2 _b2, bytes3 _b3, bytes memory _blit, bytes5 _b5, bytes6 _b6, string memory _str, bytes7 _b7, bytes22 _b22, bytes32 _b32)  {
         _b = 0x12;
         _b2 = 0x1223;
@@ -188,7 +191,7 @@ var sources = [
         _blit = hex"123498";
         _str = "this is a long string _ this is a long string _ this is a long string _ this is a long string _ this is a long string _ this is a long string _ this is a long string _ this is a long string _ this is a long string _ this is a long string _ this is a long string _ this is a long string _ this is a long string _ this is a long string _ this is a long string";
     }
-    
+
     function retunValues3 () public returns (ActionChoices _en, int[5][] memory _a1)  {
        _en = ActionChoices.GoStraight;
        int[5][] memory a = new int[5][](3);